EP0035718B1 - Process for the production of a deep-drawing sheet or strip from aluminum or - alloy - Google Patents

Abstract

Deep-drawable aluminum sheet or strip wherein the surface of a roller is blasted with broken cast steel to profile the roller, the roller is used to roll the sheet or strip to impart a microroughness consisting predominantly of steep-flank slender peaks separated by troughs thereto, and the peaks are then soft-annealed.

Description

The invention relates to a method for producing a deep-drawable sheet, strip or the like from aluminum or aluminum alloys.

It is known that when deep-drawing sheet metal, in particular of relatively thin sheet metal, the frictional relationships between the hold-down device and the drawn part are of particular importance, because they influence the flow of material into the die. In order to obtain wrinkle-free and crack-free workpieces, other parameters such as material, shape of the drawn part, drawing gap, hold-down pressure, etc. must be coordinated with one another in addition to these friction conditions. For example, the formation of folds on the drawn part cannot be prevented by an unlimited increase in the hold-down pressure, because increased frictional forces in turn lead to the formation of cracks in the drawn part. The use of lubricants also does not achieve the desired success if conventional smooth or directional surface structures are used, because in this case the lubricant film is squeezed away by the hold-down device and the friction at this point of extreme stress quickly assumes an impermissible level, which leads to the tear of the drawn part . An improvement in the friction conditions in the drawing gap and in the hold-down device is achieved if, instead of the usual blank, matt-rolled, brushed, ground or even deep-drawn foil-coated board surface, a roughened surface is used, which enables a high lubricant pressure to be built up on the tool-board contact surface. In order to obtain such a surface roughness on the deep-drawn sheet, it is known, when rolling aluminum and aluminum alloys, to have work rolls with groove-shaped depressions which are produced by grinding (AT-A-345 236) or with trough-shaped depressions which are produced by sandblasting ( AT-A-347 387) to be used to enable a large degree of rolling of 70-95%, since the lubricant is held in place by the depressions during rolling. An aluminum sheet produced in this way does not behave favorably during deep drawing, however, because the depressions in the sheet surface experience a strong longitudinal extent due to the high degree of rolling and are therefore directed. As a result, the applied lubricant can move in this direction during deep drawing. It is also known for deep-drawn steel sheets (DE-A-2 549 249) to roughen the surface between the rolls of a cold rolling mill. For this purpose, the surface of the rollers is treated using physical methods, e.g. B. by granulation, sparking or electrochemical processes. For example, 20 to 40 metal grains per mm ² in a size of 500 to 700 μm are applied to the surface of the roller in two passes for roughening. With these rollers, a surface roughness with plateaus that are as extensive as possible can then be obtained on the rolled sheet metal, a plateau height that is greater than 6 μm, preferably between 10 and 25 μm, being formed. These relatively high elevations in the form of plateaus create undesirable table mountains. In order to prepare a steel sheet for various post-processing operations, it is also known (US-A-3487674) to provide its two surfaces with different roughnesses. The surface of the sheet metal that comes into contact with deformation tools, such as drawing dies, should have roughnesses of at least 7 ghmax (Japanese industrial standard B0601), while the surface intended for coating should be noticeably smoother. For this purpose, the surfaces of the forming rollers are machined for the smooth surface by grinding and for the coarser surface by sandblasting. The grain sizes used for this in turn result in relatively high and extensive surface roughness. The knowledge gained in the manufacture of deep-drawing steel sheets could not produce any favorable results when applied to aluminum sheets.

The invention is therefore based on the object of forming the surface of an aluminum sheet or strip in such a way that improved deep-drawing results are thereby achieved.

This object is achieved according to the invention by a method for producing a sheet, strip or the like made of aluminum or aluminum alloys that can be deep-drawn and has fine micro-roughness which is as far as possible non-oriented of the sheet, strip or the like. of about 3% and the roll forming is followed by a soft annealing.

A method has proven to be particularly advantageous in which, when roll profiling, a roller whose surface is blasted with cast steel with a hardness of HRC 62-65 and grits No. to No. 4 at a centrifugal speed of 50-150 m / s is irradiated, is rolled.

For soft annealing, the strips are expediently passed through a strip drawing furnace at a speed between 30 and 90 seconds, the final temperature of the strip being 400 to 450 ° C.

It can also be advantageous that only the tips of the micro-roughness of the sheet, strip or the like are soft-annealed after roll forming.

The micro-roughness of the roll used in roll profiling can also be generated by chemical, mechanical or thermal treatment.

It has surprisingly been found that an aluminum sheet produced according to the invention with such a micro surface during deep drawing creates very favorable frictional relationships between the hold-down device and the sheet, which enables a qualified flow of the material into the die and thus largely prevents the formation of cracks and wrinkles . Such a micro-surface proves to be particularly advantageous in connection with a lubricant, because the pointed, unoriented profiles of the roughness in connection with the hold-down device seal the lubricant and prevent it from being pushed away.

• Fig. 1 einen Blechquerschnitt mit einem spitzen Oberflächen-Profil,
• Fig. 2 einen Schnitt durch das gleiche Blech, jedoch mit aufgedrücktem Niederhalter und
• Fig. 3 einen Blechquerschnitt mit einer ungünstigeren Mikrooberfläche.

An aluminum sheet or strip according to the invention is described below, which is also shown schematically in the drawing. It shows

• 1 is a sheet metal cross section with a pointed surface profile,
• Fig. 2 shows a section through the same sheet, but with pressed down and
• Fig. 3 shows a sheet metal cross section with an unfavorable micro surface.

In the illustrated sheet metal cutout 1, micro-profiles 2, which are essentially evenly distributed, rise on the surface. They have the shape of slender cones with the tips 3. The depressions 4 between the profiles are appropriately flattened or rounded in order to avoid notching effects. The micro surface is covered with a lubricant 5. As can be seen from FIG. 2, the tips of the profiles 2 are flattened by a hold-down device 7 at 8, the recesses 4 being sealed so that the lubricant filling that is absorbed remains enclosed.

In order to obtain this desired micro-surface, the invention proposes a re-rolling, the rolling surface of which is sandblasted so that a largely uniform surface is formed which, due to a slight flattening of the blasted profiles on the roller, does not result in any sharp notches in the profile base of the sheet thus rolled . A mean roughness R _a of 0.8 to 5 μm, preferably 1.5 to 3.3 μm, was determined as the favorable micro-roughness. In order to avoid an alignment or orientation of the surface structure due to the deformation, it is also important to carry out the rolling process with a reduction in thickness of at most 6%, preferably of about 3%. Especially for complicated drawn parts, where the deformability is reached, the roll pass must be carried out before soft or solution annealing, depending on the alloy used. Subsequent annealing softens the profiles hardened during re-rolling, making it easier to flatten the tips with the hold-down device. The annealing therefore meets the requirement for the softest possible characteristic of the micro-profile. The annealing should be such that its effect is aimed only at influencing the micro-profile and the mechanical-technological values of the base material are not undesirably influenced.

The rolls to be used for re-rolling the sheets are expediently sandblasted in a cabin. The roller hardness should be at least 95 Shore-C. For this, a jet of cast steel with a hardness of HRC 62 to 65 and grits No. 2 to No. 4 with a maximum grain size of 0.5 to 1.5 mm and screened fine particles can be used. The spin speeds should be 50 to 150 m / s. Irradiation takes place with the roller rotating and the beam being axially displaced.

The soft annealing is expediently carried out in a continuous strip furnace with or without protective gas. The throughput time through the furnace should be between 30 and 90 seconds, preferably 50 to 60 seconds, and the final temperature of the belt should be 400 to 550 ° C, preferably 490 to 520 ° C.

The tests to assess the improvement in deep-drawing ability by producing a micro surface were carried out with the aluminum alloys AIMg4 and AIMg Si for large-area, complex deep-drawn parts for the automotive industry with board sizes of 900 x 1550 mm. For comparison, blanks made of the same materials, in each case even from the same hot strips, but with bright-rolled and matt, and matt-rolled surface coated with thermoformed film, were used. The board thickness was 1.0 mm. All boards, including the plastic-coated ones, were greased. In the deep-drawing tests with the micro-surfaces designed according to the invention, the 10-15% lower pressure, which was necessary to produce the deep-drawn part, already showed the influence on the friction conditions. The selected, complicated deep-drawn bodies in the comparison variants resulted in 30 to 100% rejects due to crack formation, while no rejects arose from 40 deep-drawn parts with the selected micro surface, produced by rolling, R _a = 1.8 μm. Furthermore, the formation of folds, to avoid which a relatively high hold-down pressure was necessary in the comparative sheets, was significantly lower in the sheets with a micro-surface, so that the hold-down pressure could even be reduced somewhat to further increase the security against tearing.

The invention is not limited to the embodiment shown in FIGS. 1 and 2. Not all micro elevations need to have a uniform cone shape. The elevations can also differ in height, just as the cross-sectional areas parallel to the height or base axis can also be quite varied. On the other hand, it is essential that the micro-profiles are in an arrangement that is not aligned but offset against one another in the finest possible distribution tion are formed on the sheet surface. This ensures that the material flows sufficiently and evenly into the die or the die and, when using a lubricant, largely prevents it from being pushed away.

In the case of a micro profile according to FIG. 3 of the drawing with very irregularly distributed, block-like elevations 10 and with larger gaps 11, a so-called Table Mountain surface, the aforementioned advantages cannot be achieved.

Claims (5)

1. Process for the production of a deep-drawing sheet, strip or the like from aluminium or aluminium alloys having fine most possibly non- orientated micro roughnesses by being profiled by rolling with a roll whose surface has micro roughnesses with as little preferred orientation as possible, process in which the sheet, strip or the like is subjected to a reduction of thickness of approximately 3% while being profiled by rolling and this profiling by rolling is followed by annealing.

2. Process according to claim 1, in which for profiling by rolling a roll is used whose surface has been exposed to radiation with cast steel scrap with a hardness of HRC 62 to 65 and the grain sizes ranging from no. 2 to No. 4 at a centrifugal speed ranging from 50 to 150 m/s.

3. Process according to claim 1, in which for the annealing process the strips are passed through a continuous-strip furnace at a speed ranging from 30 to 90 seconds and in which the final temperature of the strip ranges from 400 to 550° C.

4. Process according to claim 1, in which only the points of the micro roughnesses of the sheet, strip or the like are annealed after having been profiled by rolling.

5. Process according to claim 1, in which the micro roughness of the roll used during the profiling by rolling is produced by chemical, mechanical orthermal treatment.

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